This invention relates to a safety glass container and more particularly, it is concerned with a strengthened safety glass container, for example, glass bottle having a high breaking strength.
Of late, synthetic resins have entered into the field of glass wares because of their having an excellent workability, a great variety of color tones and a light weight, but glass wares have still been used widely since glass is much more excellent in heat resistance and chemical resistance than plastics. However, since glass has a large weight and is often subject to breakage through even a small shock, various studies have hitherto been made so as to increase the strength of the glass.
In an example, the mechanical strength of glass is chemically increased by the so-called ion exchange method wherein an ion A contained in glass is replaced by an ion B having a larger radius. This chemical strengthening called "ion exchange method" is carried out by the spraying method as mentioned in Japanese Patent Publication No. 28674/1965 (Corning Co.), Japanese Patent Publication No. 6610/1973 (Owens Illinois Inc.) and Japanese Patent Publication No. 1316/1972 (Blockway Co.) or by the dipping method as mentioned in British Patent 917,388 (Research Corp.) and British Patent 1,010,164 (Pittsburgh Plate Glass Co.). The tensile strength of a glass container is very high, but markedly lowers if the surface is slightly scratched or abrased. Therefore, it has been proposed to protect a glass container from chances of abrasion or surface scratches by applying to the glass surface a coating of a metal oxide (Japanese Patent Publication No. 11598/1967), a polymer coating (Japanese Patent Publication No. 20716/1967) or a dual coating (Japanese Patent Publication No. 1758/1967). Furthermore, in Japanese Patent Publication No. 1307/1972 is also disclosed a method of strengthening glass, wherein a glass article is subjected to coating of a metal oxide, chemical strengthening treatment by ion exchanging and polymer coating. However, this method cannot be put to practical use in view of the complexity of the steps thereof and has another disadvantage. That is to say, the olefin polymer coating obtained by the use of an aqueous emulsion of olefin polymer as disclosed in the above mentioned specification is almost stripped in the washing step when a glass container is recovered and reused and during the same time, the surface of the glass container tends to be scratched. When reusing it, therefore, an olefin polymer coating should be formed and, as an inevitable consequence, the number of repetition thereof decreases. It has thus been desired to form a permanent resin coating such as not to be stripped in the washing step when a glass containeer is recovered and reused.
Previous attempts to protect glass articles or glass bottles from breakage are not satisfactory, such as to use a bottle shield of rubber for shock absorbing (US Pat. No. 2,706,571), to encircle the outermost surfaces of a glass bottle with rings of enamel or paper (U.S. Pat. No. 3,331,521), to encircle a bottle of thermoplastic material with a band of polyvinyl chloride in order to raise the bursting strength (U.S. Pat. No. 3,542,229), to encase the bottom and shoulder of a glass bottle in a heat-shrinkable plastic cup and cover the central portion with a polyethylene film (U.S. Pat. NO. 3,698,586) and to protect a glass article by encircling with a heat-shrinkable polyvinyl chloride film (U.S. Pat. No. 3,604,584). These proposals however aim at preventing glass articles or glass bottles from scratching in handling or shipment and reducing the breakage due to scratching as little as possible. Therefore, the strength of a glass bottle can be held as it is, but cannot be raised.